Add visual placeholders for missing book fields

[ilsea]

• Missing cover -> generate cover (already implemented)
• Missing year, number of pages -> insert symbol (-) instead
• Missing description -> "No description available for this book"
• Missing genres -> Don't display any genre chips, or "No genre available for this book"

Ref this issue comment:

We agreed on:

• English only for now.
• Author and title minimum, we can accept missing descriptions etc.
  • Frontend will implement placeholders for null values, such as - for missing year or number of pages. "Could not find a description for this book" if description is missing.
  • Missing url/cover image is already solved with a cover generator that follows the style and color scheme of the app

Originally posted by @ilsea in #59